The present invention generally relates to magnetic random access memory (MRAM) devices, and more particularly to a magnetic random access memory device having a shield member for shielding the magnetic memory cells from stray magnetic fields.
An MRAM device includes an array of memory cells. The typical magnetic memory cell includes a layer of magnetic film in which the magnetization is alterable and a layer of magnetic film in which the magnetization is fixed or xe2x80x9cpinnedxe2x80x9d in a particular direction. The magnetic film having alterable magnetization may be referred to as a data storage layer or sense layer and the magnetic film which is pinned may be referred to as a reference layer.
Conductive traces (commonly referred to as word lines and bit lines) are routed across the array of memory cells. Word lines extend along rows of memory cells, and bit lines extend along columns of memory cells. Because the word lines and bit lines operate in combination to switch the orientation of magnetization of the selected memory cell (i.e., to write the memory cell) the word lines and bit lines can be collectively referred to as write lines. Additionally, the write lines can also be used to read the logic values stored in the memory cell.
Located at each intersection of a word line and a bit line is a memory cell. Each memory cell stores a bit of information as an orientation of a magnetization. The orientation of magnetization of each memory cell will assume one of two stable orientations at any given time. These two stable orientations represent logic values of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d.
The orientation of magnetization of a selected memory cell may be changed by the application of an external magnetic field. The external magnetic field is created by supplying electrical current to a word line and a bit line which intersect at the selected memory cell. The electrical currents in the word and bit lines create magnetic fields (also referred to as xe2x80x9cwrite fieldsxe2x80x9d) surrounding the energized word and bit lines that, when combined, can switch the orientation of magnetization (and thus the logic value) of the selected memory cell. Since no electric power is needed to maintain the memory state of the device, MRAM""s are non-volatile.
Preferably, only the selected magnetic memory cell is subjected to both the word and bit line write fields. Other memory cells coupled to the particular word line preferably receive only the word line write field. Other magnetic memory cells coupled to the bit line preferably receive only the bit line write field.
The magnitudes of the word and bit line write fields are usually selected to be high enough so that the chosen magnetic memory cell switches its logic state when subjected to both fields, but low enough so that the other magnetic memory cells which are subject only to a single write field (from either the word line or the bit line) do not switch. The undesirable switching of a magnetic memory cell that receives only one write field is commonly referred to as xe2x80x9chalf-selectxe2x80x9d switching.
One problem encountered by MRAM devices is the presence of stray or external magnetic fields which emanate from sources other than the word and bit lines which are intended to write to a particular memory cell. Stray magnetic fields can originate from a multitude of sources, both within the MRAM device and external to the MRAM device. In some instances, stray magnetic fields may have a magnitude sufficient to switch the logic state of a memory cell, either as half-select switching or switching in the complete absence of a write field.
The problem of stray fields increases as the storage density of MRAM devices increases. As memory cells are packed closer together, the magnetic fields from adjacent memory cells and their associated write conductors exert a greater effect. Thus, reducing or eliminating the effects of stray magnetic fields becomes more important as storage densities increase.
A magnetic random access memory module comprises a magnetic memory array covered on a first side by a permeable metal layer. An electrically insulating layer is disposed between the permeable metal layer and the magnetic memory array. The permeable metal layer reduces the effects of stray magnetic fields on the magnetic memory array.